^ It seems like everything is mulled over, I rarely ever see a Rioter comment here, I can't say I've ever heard of anything from here being used in the actual game besides the idea for Dunkmaster Darius. Kindred was said to have had the idea created before the DhaMa concept was made, so that wasn't either. Were there any other places that ideas were used or has Riot really never used a concept from here and everything made is useless?
Dunkmaster Darius, Astro-Nautilus, the Pool Party skin line, and Surprise Party Fiddlesticks come to mind. We also know they at least like the IDEA of Dragonmaster Swain.
They've never used a champion concept though, and I doubt they will. In general I don't think the Designers spend much time on this board: they're too busy doing their own design work.
Used and given direct credit? Not much, besides some of the skins others have mentioned.
Seen and been inspired by? Almost definitely.
I heard from video from Jeremy "Gaming Curios", "What Braum could've been" that Braum was inspired by a champion concept named Angus
https://www.youtube.com/watch?v=HfC-WdoVUFY
I think Jhin was inspired by a concept named "Dimitri, the Deadeye". They have some simular things, as an example, they are marksmen and they have fixed attack speed.
While not taking the champion concepts Vel'koz, Cassiopeia, and Fiddle-sticks all have traits from champion concepts I've seen. Namely: Vel'koz's new true damage after killing them once with it, Cassiopeia's ground disable, and the new Fiddle passive. I couldn't tell you which champions they were because I read a ton of champion concepts.
Volibear was born from Champion Concept - it was a TOTAL WAR. Against the Anti-Armored Bear(Zilean and his army) and the Pro-Armored Bear(Volty and his army) it was fun to watch and in the end, we got Volibear.
Best one off the top of my head was surprise party fiddlesticks. I haven't been around that long. Also, keep in mind the creation team is constantly doing other skins and ideas- They can't always pay attention to great user artworks, sadly. You can get a little RP for it though.
The earliest example I can remember is Party Fiddlesticks skin, so yeah, they do
